(a) Field of the Invention
The present invention relates to an image transmitting optical system used for an endoscope and the like, and more specifically, to an image transmitting optical system using an optical fiber bundle.
(b) Description of the Prior Art
A conventional endoscope, for example, as shown in FIG. 1 is formed so that an image guide 2 is provided within an elongate body 1, the image of an object 0 is formed onto an entrance end face 2a of the image guide 2 by an objective 3, the image is transmitted to an exit end face 2b through the image guide 2, and the image of the exit end face 2b is formed onto an film 7 through an eyepiece 4 and a camera lens 6 in a camera body 5. And it is formed so that a light guide 8 is provided within the body 1, the light of a lamp 10 in a light source box 9 is entered an entrance end face 8a of the light guide 8 by a condenser lens 11, and the light is transmitted to an exit end face 8b through the light guide 8 to illuminate the surface of the object 0. Arranged between the exit end face 2b of the image guide 2 and the eyepiece 4 is a half prism 12 by which a part of light rays issued from the image guide 2 is transmitted onto a light receiving element 13 and the signal generated at the element 13 is led through a lead wire 14 to be transmitted to a shutter control circuit 16 for controlling the opening and closing of a shutter 15. If the distance from the objective 3 to the object 0 is long, the input signal to the shutter control circuit 16 becomes small as the surface of the object 0 is dark and consequently the speed of the shutter 15 is decreased. If the distance from the objective 3 to the object 0 is short, the input signal to the shutter control circuit 16 becomes large as the surface of the object 0 is bright and consequently the speed of the shutter 15 is increased. Therefore, the endoscope is formed so that a proper exposure may be automatically performed by the above formation. However, as this optical system performs the exposure control by changing the shutter speed, a stop aperture is always constant and the shutter speed is considerably increased in the case of the short distance photographing and consequently, as a focal depth becomes small substantially, there is a problem that the photograph is easy to be unclear.